Making informed choices: medium, serum, and serum-free medium. How to choose the appropriate medium and culture system for the model you wish to create.

Complex nutrient mixtures, which are usually called "media," are almost always supplemented with serum, with another complex biological fluid (e.g., milk, embryo extracts, and plasma), or with a defined mixture of hormones and growth factors. The choice of medium and supplements can have a major impact on the growth, function, and even phenotypic and genetic stability of cells in vitro. This choice thus becomes an important part of developing a useful and meaningful in vitro model system. This chapter defines the various roles that the medium plays in supporting cell function and outlines a method for selecting and optimizing medium in growing the cell of choice.